<button class="fullscreen-btn">
  <svg viewBox="0 0 448 512" height="1em" xmlns="http://www.w3.org/2000/svg">
    <path
      d="M32 32C14.3 32 0 46.3 0 64v96c0 17.7 14.3 32 32 32s32-14.3 32-32V96h64c17.7 0 32-14.3 32-32s-14.3-32-32-32H32zM64 352c0-17.7-14.3-32-32-32s-32 14.3-32 32v96c0 17.7 14.3 32 32 32h96c17.7 0 32-14.3 32-32s-14.3-32-32-32H64V352zM320 32c-17.7 0-32 14.3-32 32s14.3 32 32 32h64v64c0 17.7 14.3 32 32 32s32-14.3 32-32V64c0-17.7-14.3-32-32-32H320zM448 352c0-17.7-14.3-32-32-32s-32 14.3-32 32v64H320c-17.7 0-32 14.3-32 32s14.3 32 32 32h96c17.7 0 32-14.3 32-32V352z"
    ></path>
  </svg>
  <span class="tooltip">Fullscreen</span>
</button>

<style>
/* From Uiverse.io by vinodjangid07  - Tags: tooltip, button, hover effect, fullscreen, full window */
.fullscreen-btn {
  width: 25px;
  height: 25px;
  border: none;
  display: flex;
  align-items: center;
  justify-content: center;
  background-color: transparent;
  cursor: pointer;
  transition: all 0.3s;
  position: relative;
  overflow: hidden;
}

.fullscreen-btn svg {
  height: 100%;
}
.fullscreen-btn:hover {
  width: 30px;
  height: 30px;
  overflow: visible;
}
.tooltip {
  position: absolute;
  top: -40px;
  background-color: rgba(0, 0, 0, 0.753);
  color: white;
  padding: 5px 10px;
  border-radius: 5px;
  font-size: 0.8em;
  transition: all 0.3s;
  opacity: 0;
}
.fullscreen-btn:hover .tooltip {
  transform: translateY(2.5px);
  opacity: 1;
}

</style>
    